Ethylene glycol is currently made by the silver-catalyzed oxidation of ethylene to ethylene oxide, which is reacted with water to give the glycol in overall yields approaching 75% based on ethylene. Other ethylene-based processes have been used and proposed for the preparation of ethylene glycol. Such processes include the well-known halohydrin process and chloride electrochemical oxidation.
The use of synthesis gas as an alternative feedstock for the preparation of ethylene glycol has been investigated. Such a process requires pressures in excess of 16,000 psig and very high loadings of expensive rhodium catalyst (3000 ppm) to obtain modest production rates. The process produces an undesirable mixture of methanol, ethanol, propylene glycol, and traces of glycerine in addition to ethylene glycol.
Glycerine may be obtained in limited quantity by the saponification of fats. Larger quantities of glycerine can be obtained using propylene-based chemistry, such as the hydrolysis of epichlorohydrin and the epoxidation and subsequent hydrolysis of allyl alcohol.
The cleavage of vicinal diols by chemical means is well known in the art. Periodic acid cleavage has been used to unselectively degrade carbohydrate molecules to one-carbon fragments. Likewise, potassium permanganate and potassium persulfate together with silver have been used to cleave vicinal diols. The former reagent is generally not selective for producing aldehyde cleavage fragments, frequently oxidizing the primary alcohol groups to carboxylic acids. The electrochemical interactions of carbohydrate-derived polyols have been investigated sporadically in the past. The prior art teaches that glucose can be oxidized to gluconic acid by electrolysis in the presence of bromine. Also, the prior art teaches that mannitol, a carbohydrate-derived polyol, is decomposed to a mixture of formaldehyde and carboxylic acids.
Unlike the prior art discussed above, the present invention provides a method for producing ethylene glycol and glycerine from polyols which may be derived from carbohydrates. The inventive process involves selectively electrolyzing the polyols to two- and three-carbon fragments which can then be reduced to ethylene glycol and glycerine. The inventive process is advantageous in that it allows the preparation of key raw materials totally from renewable resources. The feed to the process comprises carbohydrate-derived polyols which can be obtained by the acidic or enzymatic hydrolysis of cellulose or starch or from naturally occurring monosaccharides. It is contempated that the present invention is especially suitable for use in a solar-hydrogen-electric economy.